Technical Field
The present invention relates to an apparatus for mooring a floater using a submerged pontoon and, more specifically, to an apparatus for mooring a floater using a submerged pontoon that secures a floater such as floating production storage and offloading on the surface of the sea, the depth of which is about 50 m or more below sea level or deep sea.
Background Art
In drilling and producing offshore petroleum and gas, a floater such as floating production storage and offloading (hereinafter referred to as “FPSO”) is to be moored on the surface of the sea, the depth of which is 50 m or more below sea level or deep sea, for which various mooring systems are referred to in related patent documents.
1. Spread Mooring: A mooring system that is mostly applied in the shallow sea and is difficult to disassemble and reconstruct after the initial construction. This is a technology to fix and moor a floater by dropping anchors and anchor ropes directly to the seabed from the floater such as FPSO, wherein a plurality of anchors and anchor ropes are radially distributed with the floater such as FPSO at the center of anchors and anchor ropes.2. Internal Turret Mooring: A mooring system that pierces a hole on the bottom surface of a floater such as FPSO to install an internal turret from which a plurality of anchor ropes are radially lashed in order to connect the mooring ropes to the floater. The turret that lashes the mooring ropes is also used as a connecting path for the petroleum production piping of the oil well and, for this purpose, a swivel that is a rotating joint is installed. A swivel is expensive and, upon a failure or damage, may cause an accident such as crude oil leakage thus produced. The floater rotates on the turret in the direction of wind, avoiding cross winds in heavy weather, thus effectively coping with stormy weather. However, in extremely heavy weather, the floater is to be separated, for protection, from the turret and kept out of the mooring system. It requires a relatively long time and considerable efforts to reassemble the turret when the floater returns to the operation area after stormy weather.3. External Turret Mooring: A mooring system in which the internal turret employed in an internal turret mooring system is to be installed frontward outside a floater such as FPSO. This is an expensive rotating equipment for the purpose of connecting and mooring the petroleum production piping of the oil well and poses a chance for leakage at the swivel that is a rotating joint. This system is difficult, after the initial construction, to disassemble, recover and reassemble for an emergency4. Catenary Anchor Leg Mooring (CALM): A mooring system that lashes a floating structure with a plurality of anchor ropes that radially extend to secure the floating structure at a position and moors a floater such as FPSO to the floating structure. This is economic but does not firmly secure at an exact position.5. Single Anchor Leg Mooring (SALM): Similar to a CALM abovementioned, but lashes a floating structure with a single anchor rope and moors a floater such as FPSO to the floating structure. This is economic as well as easy to operate in the deep sea, but hard to moor at an exact position.6. Dynamic Positioning (DP): Instead of external lashing equipment such as anchor rope, this system utilizes, beneath the bottom of a floater such as FPSO, a plurality of thrusters that can rotate 360 degrees, receives positioning data from satellites and continuously controls the direction and speed of the thrusters in order to keep the floater at a constant position. Although propitious in the deep sea, it requires a high amount of costs for installation and continuous operation.7. Tower Mooring: A system available only in the shallow sea that builds up an offshore tower founded in the seabed and moors a floater such as FPSO to the tower.
As a non-patent literature such as a technical report, issued by Civil Engineering Laboratory (USA) describes a mooring system that submerges individual buoys in the sea. The system is to secure a smaller installation, instead of a floater, such as buoy at a certain position.
As described, existing mooring methods are difficult, after constructing a floater such as FPSO on the sea surface, to separate the floater off the position including collision avoidance in a short period when an emergency occurs such as stormy weather and requires a considerable amount of time, after the emergency, in returning to the original position and reconstructing the floater. A mooring system according to CALM or SALM technique is easy to operate but disadvantageous in keeping an exact position and developing an oil well of a large scale. Furthermore, those systems require high costs according to the species of mooring equipment, and, if employing rotating crude oil production piping, are vulnerable to leakage of the crude oil thus produced, materials reinjected to the oil well, etc.